The present invention relates to free piston engines.
Conventionally, internal combustion engines have operated with the motion of the pistons mechanically fixed. For example, a conventional internal combustion engine for a motor vehicle includes a crankshaft and connecting rod assemblies that mechanically determine the motion of each piston within its respective cylinder. This type of engine is desirable because the position of each piston is know for any given point in the engine cycle, which simplifies timing and operation of the engine. While these conventional types of engines have seen great improvements in efficiency in recent years, due to the nature of the engines, that efficiency is still limited. In particular, the power density is limited because the mechanically fixed motion of the pistons fixes the compression ratio. Moreover, all of the moving parts that direct the movement of the pistons (and camshafts and engine valves as well) create a great deal of friction, which takes energy from the engine itself to overcome. The resulting lower power density means that the engine will be larger and heavier than is desired. Also, the flexibility in the engine design and packaging is limited because of all of the mechanical connections that must be made.
Consequently, is desirable, for environmental and other reasons, to have an engine with a higher power density than these conventional engines. The advantages of lighter relative weight, smaller package size, and improved fuel efficiency can be a great advantage in both vehicle and stationary power production applications.
Another type of internal combustion engine is a free piston engine. This is an engine where the movement of the pistons in the cylinders is not mechanically fixed. The movement is controlled by the balance of forces acting on each piston at any given time. Since the motion is not fixed, the engines can have variable compression ratios, which allow for more flexibility in designing the engine's operating parameters. Also, since there are no conventional crankshafts and rods attached to the crankshaft, which reduces piston side force, there is generally less friction produced during engine operation. However, these types of engines have not come into common use because, with free pistons, the complexity of engine operation is greatly increased.
One concern, in particular, is assuring sufficient heat transfer from each piston to its cylinder wall. Without this, there may be locations of overheating on the free piston assembly. Crankshaft engines inherently induce a side loading of the pistons, which is reacted against the cylinder walls. The contact induced by this side loading allows for significant heat transfer from each piston to its cylinder wall. But in a free piston engine, it is undesirable and unnecessary that there be side loading, thus eliminating the contact between the piston skirt and the cylinder wall. While this reduces the friction between the piston and cylinder and reduces the amount of lubrication oil needed on the cylinder walls, it also reduces the contact area for transferring heat. The ability to adequately cool a piston is especially important for engine configurations where there is a piston that operates adjacent to that cylinder's exhaust port.